This invention relates to a process for controlling the charging of a shaft furnace of the type which utilizes a rotary or oscillating distribution spout for distributing charge material over the charging surface of the furnace. One or more hoppers for storage of the charge material are located above the furnace. Each hopper is provided with a dosing device for regulating the flow of charging material from the hopper to the spout; and a weighing system for determining the contents (weight) of the hopper. This known process determines (by calculation or by experiment), the extent to which the dosing valve must be initially opened for the contents of a hopper to flow out within a given time period, and retains the theoretical curves of a given constant flow in memory for different types of material and different charging conditions. This process also retains, in memory, the corresponding position of the dosing valve required to ensure the ouflow within the given period. These curves indicate the reference flow Q.sub.c and the position of the valve at each selected moment. The real flow Q.sub.r is determined at given intervals by measuring the reduction of weight .DELTA.P in the contents of the hopper per unit time .DELTA.t; and comparing the real flow Q.sub.r with the reference flow Q.sub.c.
The charging of a shaft furnace by means of a distribution spout is typically carried out so as to deposit a diametrically symmetrical and circularly uniform layer on the charging surface using charge material eminating from a storage hopper. For this purpose, there is generally a predetermined charging period available which is governed by (1) the yield and capacity of the furnace; (2) the method of distribution; and (3) the coordination of the operations, such as the opening and closing of the valves, the transport of the charging material to the required position, etc.
When this available time period is known, the opening of the dosing valve controlling the outflow from the hopper has to be regulated such that the hopper will be empty at the moment in which the spout has swept over its complete trajectory i.e. (at the expiration of the required time period).
For this purpose, the dosing valve is regulated in the manner described above and also discussed in U.S. Pat. Nos. 3,929,240 and 4,074,816 both of which are assigned to the assignee hereof and incorporated herein by reference. In theory, an adjustment of the dosing valve as described above should make it possible to deposit the exact layer of charge material required by the smelter. Unfortunately, in practice, this is not the case because certain parameters may influence the flow regardless of the position of the valve. For example, when the degree of opening of the valve is selected on the basis of memorized standard (known) data and the nature of the material to be charged, in order to obtain a certain given rate of flow, it has been found that at the beginning of the flow phase, the weight of the column of material above the discharge aperture may increase the flow rate. On the other hand, as and when the hopper empties, the flow rate decreases as a result of the reduction in the weight of the hopper material; the rate of flow thus being reduced to below the referenced flow rate. As the operation is slowed up in this matter, the period within which the contents of a hopper has to be discharged into the furnace is inevitably exceeded. This not only upsets the charging program, but also prevents the charge layer from remaining symmetrical. Variations of the charge thus occur in the height of the material around the circular trajectory of the charging surface. The flow may also be affected by further factors such as the degree of humidity or the grain size of the material.
In order to remedy this drawback, attempts have been made to correct the position of the dosing valve in accordance with the fluctuations in the rate of flow, i.e., by slightly closing it when the real flow, as measured by the reduction in the weight of the hopper, exceeds the reference flow (opening it wider when the flow falls below this latter value). In reality, however, the flow corresponding to a certain position of the valve can only be determined after this position has been actually reached, and in view of the lapse of time involved in determining the flow, the ideal position or reference position of the valve when positional correstions are being carried out is invariably reached before this can be known. In other words, whatever the direction the valve moves, i.e., whether it opens or closes, it will always be displaced by an excessive amount, so that it then has to be corrested by moving it in the opposite direction. The result is that the real rate of flow constantly fluctuates about the reference value. This result is discussed in more detail hereinafter with reference to FIG. 2.
The only favorable result obtainable with the process described above is that it permits accurate adherence (more or less) to the time required for the discharge of the contents of a hopper. On the other hand, due to the fluctuations in the rate of flow, the deposit of the charging material is rendered still more uneven than if no corrections were made. Moreover, this process involves an additional inconvenience inasmuch as the reversals of the displacement of the valve, as it opens and closes in alternation, cause jerky moments leading to "false impulses" in the weight measuring systems.